[802.3ae_Serial] Tx setup window with OMA
As I wrote two days ago, the setup window for the LR/LW, 1310 nm serial,
PMD, at low extinction ratio is 4 dB when 5 dB was intended.
For the ER/EW, 1550 nm serial, PMD, the window is around 3.5 dB if TPD
(transmitter and dispersion penalty) is small and extinction ratio is its
minimum 3 dB, or 0.4 dB (!) at maximum TPD, minimum extinction ratio.
For the SR/SW, 850 nm serial, PMD, the window at 3 dB extinction ratio is
very roughly 2 dB; it depends strongly on the triple trade off.
The attachment Pave_OMA_LX.pdf is an example of an old style (mean power
based) standard that works: the 1 Gigabit Ethernet LX standard. The setup
window is 8 dB wide. Because both the upper and lower limits are measured
on the same basis, the window is 8 dB wide for any extinction ratio. (P1
means the power in the ones.)
At 10 Gbit/s we don't want to set a very high receiver overload value, we
want to enable lower extinction ratios, and to enable cost-effective
transmitters we wish to specify them on an OMA basis. The attachment
Pave_OMA_LRW.pdf shows where the LR/LW, 1310 nm serial, PMD is at in draft
3.1. The setup window is 4 dB wide at 4 dB extinction ratio.
How can we fix this?
Options are:
1. Reduce the Tx OMA spec. Would need better receivers and/or reduced
link attenuation. Not attractive.
2. Raise the Tx mean power spec back to 1 dBm. This works.
3. Specify the Tx maximum power on a maximum-in-in-the-ones basis at
e.g. +3 dBm. This seems to work too. It's a reasonable compromise between
the transmitter's primary metric (OMA) and the receiver's overload concern
(power in the ones? mean power? OMA? it may depend on receiver
implementation).
4. Specify Tx maximum in OMA, at say +1 dBm. This makes the setup
window independent of extinction ratio again but it gives the receiver a
hard time.
5. A combination of 2 and 3 above, as illustrated in purple. Not sure
that the benefit over the better of options 2, 3 outweighs the cost of the
extra test involved.
Option 2 isn't an original idea. It was proposed for 1550 nm in
http://www.ieee802.org/3/ae/public/jan01/frojdh_2_0101.pdf and shelved
because it wasn't essential to fix anything broken and we wanted to know
more about how 1550 nm receivers overload before taking it up.
Piers
Pave_OMA_LX.pdf
Pave_OMA_LRW.pdf